(1) Field of the Disclosure
The invention relates to improvements in methods of making security features, in particular electrotype security features.
(2) Description of the Related Art
The electrotype is not a new security feature; effectively it is a crude watermark that has been known for over 100 years. An electrotype is a thin piece of metal in the form of an image or letter that is applied to the face cloth of the cylinder mould of a papermaking machine, by sewing or more recently welding, resulting in a significant decrease in drainage and fibre deposition forming a light mark in the paper. This type of process is well known in papermaking and is described in U.S Pat. Nos. 1,901,049 and 2,009,185.
DE-A-102005042344 discloses a dewatering screen for the production of paper having multi-layered watermarks, with a support screen and a perforated watermark metal sheet connected to the support screen, in which the support screen and the watermark metal sheet are embossed jointly in the form of the watermark to be produced.
One method of producing electrotypes utilises a standard electroplating process. An image is prepared in wax, which is then sprayed with silver. Copper is then deposited on the wax to form the electrotype, which is separated from the wax base with hot water. A number of problems exist with this process:                1. The process is difficult to control and a constant thickness could not be maintained across the electrotype. This results in the final image in the paper appearing non-uniform with variable intensity;        2. Poor resolution;        3. Expensive labour intensive process.        
The electrotype is typically attached to the face cloth by resistance welding. Welding tips of different diameters are available in the range 0.8 mm to 3 mm. The welding tip is placed on the electrotype with the heat transferring through the electrotype to the face cloth. The welding process becomes increasing difficult as the tip size is reduced below 2 mm, with the smaller tips resulting in distortion and an uneven surface. Practically it is not possible to weld with a tip smaller than 0.8 mm.
The papermaking process also places design constraints on the electrotype. The line width of an electrotype image is preferentially in the range 0.3-1.1 mm. Increasing the line width above 1.1 mm usually results in pinholing. This is the situation where there are insufficient fibres formed over the electrotype to form a visually continuous layer of fibres resulting in discernible holes in the paper. The minimum line spacing achievable is 0.25 mm, anything less than this is not resolvable in the final paper. If the spacing cannot be resolved the result is an increased line width that leads to pinholing.
A further limitation to the resolution of the electrotype is the size of the face cloth mesh. The typical mesh size for a face cloth is given below:                Warp (lines around cylinder)—70 wires per inch (25.4 mm), 0.2 mm diameter, 0.25 mm gap        Weft (lines across cylinder)—48 wires per inch (25.4 mm), 0.2 mm diameter, 0.4 mm gap.        
FIG. 1 shows three different circular electrotypes 10a, 10b, 10c of diameter 0.3 mm, 0.5 mm and 1 mm positioned on the wire mesh of a face cloth 5. In the case of the electrotype 10a formed by the 0.3 mm circle, there is negligible overlap between the warp and/or weft of the face cloth 5 and the electrotype 10a and it is therefore very difficult to securely weld the electrotype 10a to the face cloth 5. It becomes increasingly easier to obtain large enough areas of overlap as the diameter increases to 0.5 mm and 1 mm respectively as shown on the diagram by electrotypes 10b and 10c respectively.
A further problem with electrotypes is shown in FIG. 2 and relates to the generation of complex designs with unconnected elements 6. Unconnected elements 6 have to be joined with unsightly tie lines 7. The tie lines 7 are necessary because the unconnected elements 6 are too small and intricate to weld accurately in position even if the size of the unconnected elements 6 is greater than the diameter of the welding tip. The tie lines 7 effectively create one single electrotype that can be accurately positioned and welded. It is then necessary to remove the tie lines 7 before the face cloth 5 is used, this becomes very difficult and in some cases impossible when the design is very intricate. In this case the tie lines 7 are left in place and form an unwanted part of the design.
It is therefore an object of the present invention to provide an improved method of making an electrotype security feature which resolves the above described problems.
According to the invention there is provided an electrotype for attachment to the face cloth of a cylinder mould for forming an image during a paper making process, the electrotype comprising a mesh and at least one image forming element attached to the mesh.
The invention further provides a method of forming an electrotype as claimed in any one of the preceding claims comprising the steps of electroforming a first layer comprising a mesh and at least one image forming element.